A mitered binding or a mitered border? If it's the binding, do what soccertxi suggested and try folding it the other way. If it's a border, I would first undo the seam. If you pressed your seam to one side, one side will still be pressed flat. Just lay the flat side on the bottom, then the side with the creased pressed seam, smooth, carefully pin and re sew. This might be a good place for (horrors) basting first. If you pressed your seam open, then unpress one side and do the same thing.